One group, the non-profit B612 Foundation, proposes sending a telescope, called Sentinel, into space to detect incoming objects decades before their orbits intersect ours. Then, unmanned spacecraft could fly to them and nudge them clear of Earth's path.

The group is trying to raise $200 million to make it happen and hopes to launch the telescope by 2016.

Another project, proposed by the University of Hawaii, aims to give earthlings a heads-up when necessary, starting by 2015.

It is called the Atlas program, and the plan is to deploy a string of telescopes that would search for even smaller objects in the sky, hoping to be able to give people at least a few day's notice that could allow time for an evacuation.